The reason you have not found much on this on these forums, is because this questions are in WPMU’s domain. Have a search on their forums as these components aren’t provided or controlled by BuddyPress.

You’ll want to turn off blog registration and also set the default user role in WPMU to be “author”; that way everyone will be able to make a post on the root blog.

You can do this by going into your WPMU admin area and navigating to “Site Admin > Options”. Set “Allow New Registrations” to “Only user account can be created”. And set “Dashboard User Default Role” to “Author”.

For #2:

You’ll want to use some type of customized post screen plugin such as Flutter or my personal favorite Custom Field Template.

That should get you going!

You might also want to read this thread which talks about how to create a community blog.